Methods of and/or means for indicating the levels of liquids

ABSTRACT

A wire wound resistor is vertically disposed in a vessel. The resistor has a former of between 20 mm and 45 mm diameter and resistance wire of a resistance of between 1 and 4 ohms per turn connected in the circuit of an A.C. bridge-circuit. Impedance change is indicated on a display unit calibrated to indicate the change in level of liquid e.g. milk in the vessel. The resistance wire is laid on the former by applying ultrasonic sound energy to soften the former which is of polysulfone or polycarbonate and the wire embedded in the softened former material. The vessel is shaped at the lower end to assist in giving linear readings when the level of liquid is low.

BACKGROUND AND SUMMARY OF THE INVENTION

This invention relates to methods of and/or means for indicating thelevels of liquids and has been devised particularly though not solelyfor use in measuring the level of milk in a vessel.

It is an object of the present invention to provide methods of and meansfor measuring the level of liquid in a vessel which will at leastprovide the public with a useful choice.

Accordingly in one aspect the invention consists in a method ofindicating the level of an electrically conducting liquid in a vessel,said method comprising the steps of allowing the liquid to rise in thevessel which contains a vertically disposed rod type resistor having anaverage cross sectional dimension between twenty and forty five mm and aresistance of between three and thirty two ohms per mm of length of theresistor, said method comprising the steps of allowing the liquid torise in the vessel in a manner such that the liquid interconnectsadjacent lengths of the resistor thus changing the impedance of theresistor appearing at the terminals thereof, measuring that change ofimpedance and converting the change of impedance into a display whichindicates change of level in liquid in the vessel.

In a further aspect the invention consists in apparatus for indicatingthe level of an electrically conducting liquid in a vessel saidapparatus comprising a rod type resistor disposed vertically in use insaid vessel, said resistor having an average cross sectional dimensionbetween twenty and forty five mm, and a resistance of between three andthirty-two ohms per mm of length of the resistor and being arranged sothat increase in the level of liquid in the vessel causes some of theliquid in the vessel to interconnect adjacent lengths of the resistancewire to change the impedance thereof, means for measuring such change inimpedance and display means on which change in level of liquid in thevessel is indicated.

To those skilled in the art to which this invention relates, manychanges in construction and widely differing embodiments andapplications of the invention will suggest themselves without departingfrom the scope of the invention as defined in the appended claims. Thedisclosures and the description herein are purely illustrative and arenot intended to be in any sense limiting.

BRIEF DESCRIPTION OF THE DRAWINGS

One preferred form of the invention will now be described with referenceto the accompanying drawings in which:

FIG. 1 is an elevation of a resistor for use in apparatus according tothe invention,

FIG. 2 is an enlargement of two turns of wire arranged according to FIG.1,

FIG. 3 shows the resistor of FIG. 1 disposed in a shaped vessel.

FIG. 4 is a part figure as FIG. 1 showing different terminalconnections,

FIG. 5 is a plan view of a resistor winding arrangement,

FIG. 6 is a sketch elevation of the construction shown in FIG. 5 and,

FIG. 7 is a part section of the wall of a resistor former withresistance wires shown embedded therein.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings, apparatus for indicating the level of anelectrically conducting liquid in a vessel is provided for establishingthe level of milk supplied from an individual cow for the purpose ofchecking on the performance of that cow.

Accordingly a rod type resistor is constructed comprising, for example,an insulating former 1 which is preferably closely wound with a finegauge resistance wire 2. The former is of an average cross sectionaldimension between twenty and forty-five mm, preferably twenty-five mmand is preferably cylindrical although other shapes which give a similarsurface area may be used.

The unit described herein employs thirty-four to thirty-eight SWGnichrome wire and when such wire is wound on the twenty-five mm diameterformer 1 for about four hundred mm of length, it has a total resistanceof approximately four thousand ohms. The turns are closely spacedgiving, for example, between three and eight turns per mm preferablyfive turns per mm and this gives a resistance of between one and fourohms per turn, typically about two ohms per turn. The range ofresistance preferably lies between three and thirty-two ohms per mm oflength.

An insulated lead 3 connects the lower end of the winding 2 to aterminal 4 and the upper end of the element 2 is connected to a terminal5. The resistance of the element is measured by means of a bridgecircuit e.g. a Marconi AC Wheatstone Bridge using alternating current atapproximately 1000 Hz. Use of an alternating current preventspolarisation at the surface of the wire when immersed in milk. Thebridge circuit supplies a display means in which the resistance iscalibrated to give an indication of liquid level in a vessel (not shownin FIG. 1). With the apparatus described the element of 4000 ohms wasreduced to an impedance of about 1500 ohms by complete immersion inmilk. This reduction in impedance appearing at the terminals 4 and 5 isdue to the milk, being a conducting liquid, allows current to flow fromthe exposed wire surface through the milk so that the turns ofresistance wire below the liquid level are effectively in parallel withthe body of milk.

Amongst other liquids of the same nature milk forms a sticky film whichwill not run off surfaces readily and thus the level indication withprior art measuring elements is usually higher than the actual liquidlevel once the element has been immersed and withdrawn again. Thebutterfat content of the milk may cause a film of fat to accumulate onthe wire and thus increase the resistance of the contact between thewire and the milk. The calibration of an element not using the presentinvention can thus be extremely variable. In particular if a resistorcomprising a single wire strand is used to measure the level of milk ina vessel the shunting of the single wire by the poorly conducting milkis very small so that extremely small changes of resistance must bedetermined. The present invention at least in the preferred form avoidsthese difficulties by the use of:

(a) The larger diameter of from twenty to forty-five mm for the former

(b) The use of a fine resistance wire which results in a high totalresistance of wire but because there are many turns a relatively lowchange in resistance per turn, namely of the order of one to four ohmsas mentioned above. The total area of the wire in contact with the milkis thus sufficiently great to allow a good low resistance connection tothe milk in contact with it. A film on non-immersed turns has littleeffect as the milk film represents a very high resistance cross sectionwhich is in parallel with the relatively low resistance of the wireitself. Thus referring to FIG. 2 in which the resistance per turn ofwire is two to four ohms approximately, if the resistor has beenimmersed in milk and then withdrawn or more likely the liquid level hasbeen reduced in the vessel, a film of milk represented by the crosses inFIG. 2 will present a high parallel resistance so that the shunt effectto each turn is very small and even the shunt effect over several mm oflength of film is still not great. Furthermore because there are manyturns of wire on the surface of the former 1 and such turns are exposedto the milk a considerable surface area of wire is available to providelow contact resistance between the milk and the turns of wire below thelevel of the milk. This is brought about by the large diameter of theformer 1 and by the large number of turns of wire on that former. Anadded effect is that because there are many turns a very small change inliquid level can be detected. Expermentation has shown that theindication of milk level was substantially unaffected by the presence ofsurface contamination by milk fat and liquid milk or a form of milk.Even when the surface was coated slightly with petroleum jelly theindication remained correct. Two coats of polyurethane spray werenecessary to prevent the element from providing a satisfactory workingresult. Thus the advantages of the invention stem from:

(a) The use of a large diameter former providing a long length ofresistance wire to make contact with the milk in which it is immersed,results in a large contact area and even though the unit contactresistance between the wire and the milk may be high, the large totalcontact area results in a low resistance connection, to a thick (say 10mm) layer or annulus of milk around the rod type resistor.

(b) At the same time the resistance of individual turns of the elementis very low (one to four ohms) so that the resistance of a ring of milkbetween wires over the distance of one turn i.e. in the film has littleeffect as the milk represents a very high resistance due to its smallcross section and this high resistance is in parallel with the one tofour ohms of the turn of wire.

(c) The large area of resistance wire exposed to the milk provides a low"resistance of contact" to the milk because

1. It provides a large area contact and

2. The resistance of the metal to unit area is quite low. Provided thevessel containing the milk is of great enough cross section to provide athick, say 10 mm, layer or annulus of milk around the resistor, theparallel resistance of that body of milk is low enough to reduce theeffective resistance of that portion of the resistor immersed in themilk by approximately 70%. This would not be the same with a singlestand or narrow diameter element or a small number of turns of wirewidely spaced on a narrow diameter former.

The effects of the invention are augmented by the use of an alternatingcurrent in the element. This alternating current has probably twoeffects, firstly it inhibits polarisation effects on the milk andsecondly there is probably a capacitive coupling between the wires andthe liquid even in the presence of a thin film of butterfat or othermilk material. Thus there is a change in impedance of the resistorrather than a change in resistance only.

Where the liquid being measured (such as milk) varies in conductivityover a small range the resistance change with immersion will be affectedby the conductivity to a small degree. If necessary this can becorrected by detecting the conductivity of the liquid and using this asthe basis to suitably adjust the electrical or electronic measuring orconverting system. Thus referring to FIG. 4 it is possible to providetwo small conducting terminals for example, stainless steel studs 8 & 9at a lower end of the resistance element former 1. One of these studs 8is connected to the lower end of the resistance element and the otherstud 9 is connected to a separate insulated lead to a terminal 10 andthence to the measuring system. An added advantage of this conductivitysensing system is the capability of using it to control an indicatorsystem for monitoring the conductivity of the liquid. It can then bearranged, for example, to give an indication of cell count (mastitislevel) as conductivity of milk may be directly related to mastitislevels. In regard to the detection of mastitis reference should be madeto our New Zealand Pat. Nos. 180087 and 188810, respectivelycorresponding to U.S. Pat. Nos. 4,156,179 and U.S. .[.application No.982..]. .Iadd.Pat. No. 4,309,660, issued Jan. 5, 1982. .Iaddend.

Because the change in resistance with immersion departs from linearrelationship when very little of the resistance element is immersed, itis necessary or desirable to make a correction near the zero level.

Where the level is being used to measure volume of liquid, this can bedone in various ways.

1. By shaping the resistance element former to give a change inresistance per unit length,

2. By varying the spacing of the resistance element winding at the lowerend,

3. By shaping the containing vessel to alter relationship between volumeand depth of liquid. The method described in item 3 above is the mosteasily applied and is illustrated in FIG. 4. Where the containing vessel11 is provided with frustums of cones 12 and 13 having different slopesso that the liquid level rises more rapidly in the lower section 13 thanin the upper section 12. The taper profiles are determinedexperimentally.

It will be apparent from the foregoing that the very fine wire withwhich the resistor is wound is somewhat fragile and therefore needs someprotection. It is also necessary to provide a resistance element with asmooth surface without irregularities which could be difficult to cleanand could provide areas for bacteria to collect. In FIGS. 5 & 6 there isillustrated a suitable winding machine comprising a frame 20 and a drivemechanism (not shown) driving a shaft 21 which rotates the former 1. Thewire is led through a guide and tensioning means 22 which is moved alongthe length of the former 1 by lead screw gears 23 driving a lead screw24 to give the desired pitch to the wire. An ultrasonic horn 25 isarranged to feed ultrasonic sound energy onto the wire where it firstcontacts the former which is, for example, made of a material such aspolysulfone or polycarbonate. The wire heats and softens the former onmaking contact therewith. A fixed supporting anvil 26 is mounted belowthe ultrasonic horn 25 and this is made of a suitable energy absorbingmaterial such as high density polyethylene. The effect of the ultrasonichorn is illustrated in FIG. 7 where previously wound turns 27 of wireare shown embedded in the former 1 and a turn 28 is shown above thelevel of the former 1 but with the former 1 softened therearound so thatby the application of tension applied by the guide and tensioning means22 the wire is pulled in to the surface of the former so that while anadequate surface area 29 is provided to enable the action of theinvention to take place, yet the wire is adequately protected againstwashing or other procedures likely to cause damage thereto and thesurface is relatively smooth for washing.

As mentioned above the vessel 11 is a flask of a milk meter such as theWaikato Mark III milk meter manufactured by AHI Plastic MouldingCompany. The dimensions of the flask are not critical but of course willhave an effect on the accuracy of measuring the volume of liquid. It ispreferable that the walls of the vessel 11 are not closer than 10 mm tothe wire on the former 1. Beyond this there is no real limit to thedistance of the walls of the vessel from the former 1 except as statedthat the greater this distance the more volume there will be for a givenchange of height and thus as stated the accuracy will be affected.

The present invention provides a simple interface between a standardmilking system by providing means to give an electrical signal inproportion to the depth of milk in a flask or vessel which reflects aproportion of the total output from a particular cow. Previous attemptshave been made in this regard on a number of occasions with varyingdegrees of success.

What is claimed as new is as follows:
 1. A method of indicating thelevel of an electrically conducting liquid milk in a vessel whichcontains a vertically disposed rod type resistor having adjacent lengthshaving an average cross sectional dimension between twenty andforty-five mm and a resistance of between three and thirty-two ohms permm of length of the resistor, said method comprising the steps of (a)allowing the liquid .Iadd.milk .Iaddend.to rise in the vessel in amanner such that the liquid .Iadd.milk .Iaddend.interconnects theadjacent lengths of the resistor thus changing the impedance of theresistor appearing at the terminals thereof, (b) measuring .[.that.]..Iadd.said .Iaddend.change of impedance and (c) converting .[.the.]..Iadd.said .Iaddend.change of impedance into a display which indicateschange of level of liquid .Iadd.milk .Iaddend.in the vessel, wherebysaid dimension enables the total area of the resistor in contact withthe .Iadd.liquid .Iaddend.milk to be sufficiently great to allow a goodresistance connection to the .Iadd.liquid .Iaddend.milk in contact withit and enables extremely small changes of impedance to be measured instep (b).
 2. A method as claimed in claim 1 wherein step (a) is carriedout by using a wire wound resistor having between three and eight turnsper mm and a resistance of between one and four ohms per turn ofresistance wire and so that the liquid .Iadd.milk .Iaddend.interconnectsadjacent turns of the resistor.
 3. A method as claimed in claim 1wherein the change of impedance in step (b) is measured by applying analternating current to the terminals of the resistor and measuring thechange in alternating current flowing through the resistor.
 4. Apparatusfor indicating the level of an electrically conducting liquid milk in avessel, said apparatus comprising (a) a rod type resistor.Iadd.including a former having resistance wire wound thereon and.Iaddend.disposed vertically in use in said vessel, said resistor havingan average cross sectional dimension between twenty and forty-five mmand a .[.resistor.]. .Iadd.resistance .Iaddend.of between three andthirty-two ohms per mm of length of the resistor, the resistor beingarranged so that increase in the level of liquid .Iadd.milk .Iaddend.inthe vessel causes some of the liquid in the vessel to interconnectadjacent lengths of the resistor to change the impedance thereof, (b)means for measuring .[.such.]. .Iadd.said .Iaddend.change in impedanceand (c) display means on which change in level of liquid .Iadd.milk.Iaddend.in the vessel is indicated.Iadd., whereby said resistance andsaid dimension enable the total area of the resistor in contact with theliquid milk to be sufficiently great to allow a good resistanceconnection to the liquid milk in contact with it and enables extremelysmall changes of impedance to be measured by said means formeasuring.Iaddend..
 5. Apparatus as claimed in claim 4 wherein saidresistor is a former wound with wire having between three and eightturns per mm and a resistance of between one and four ohms per turn andbeing arranged so that increase in the level of liquid .Iadd.milk.Iaddend.in the vessel causes some of the liquid .Iadd.milk .Iaddend.tointerconnect adjacent turns of said wire.
 6. Apparatus for indicatingthe level of an electrically conducting liquid in a vessel, said vesselcomprising (a) a rod type resistor disposed vertically in use in saidvessel, said resistor having an average cross sectional dimensionbetween twenty and forty-five mm and a resistance of between three andthirty-two ohms per mm of length of the resistor, the resistor beingarranged so that increase in the level of liquid in the vessel causessome of the liquid in the vessel to interconnect adjacent lengths of theresistor to change the impedance thereof, (b) means for measuring.[.such.]. .Iadd.said .Iaddend.change in impedance and (c) display meanson which change in level of liquid in the vessel is indicated, whereinsaid resistor is a former wound with wire having between three and eightturns per mm and a resistance of between one and four ohms per turn andbeing arranged so that increase in the level of liquid in the vesselcauses some of the liquid to interconnect adjacent turns of said wire,wherein said former is made of a softenable material such as polysulfoneor polycarbonate and said resistance wire is embedded in said softenablematerial of the former with only a portion thereof exposed, whereby saiddimension enables the total area of the wire in contact with the liquidto be sufficiently great to allow a good resistance connection to theliquid in contact with it and enables extremely small changes ofimpedance to be measured.
 7. Apparatus as claimed in claim 5 whereinseparately spaced studs or terminals are provided on said former andwhereby the conductance of the liquid the level of which is beingmeasured, can be measured by a conductance measuring device. 8.Apparatus as claimed in claim 5 wherein said vessel is provided with aplurality of frusta of cones at the lower end thereof, the lowermost ofsaid plurality having a lesser slope than the next frustum for thepurpose of assisting in giving a linear scale during the commencement offilling of the vessel. .Iadd.
 9. A method of indicating the level of anelectrically conducting liquid in a vessel, said method comprising thesteps of (a) allowing the liquid to rise in the vessel which contains avertically disposed resistor having a conducting surface on aninsulating former rod having an average perimeter in excess ofapproximately 63 mm, the resistor having a resistance between three andthirty-two ohms per mm of height of the resistor and comprising a wirewound resistor having in excess of three turns per mm and a resistanceof between one and four ohms per turn of resistance wire and so that theliquid interconnects adjacent turns of the resistor which are immersedin the liquid, the liquid rising in the vessel in a manner such that theinteraction of the liquid and the resistor reduces the impedance of theresistor appearing at the terminal thereof, (b) measuring said reductionof impedance at said terminals, and (c) converting the reduction ofimpedance into a display which indicates rise of level in liquid in thevessel. .Iaddend. .Iadd.10. A method as claimed in claim 9, wherein step(a) is carried out by using a resistor having an average perimeter ofless than 140 mm. .Iaddend. .Iadd.11. A method as claimed in claim 9,wherein step (a) is carried out by using a resistor having up to 8 turnsper mm. .Iaddend. .Iadd.12. A method as claimed in claims 9, 10 or 11,wherein the reduction of impedance is measured in step (b) by applyingan alternating current to the terminals of the resistor and measuringthe change in alternating current flowing through the resistor..Iaddend. .Iadd.13. Apparatus for indicating the level of anelectrically conducting liquid such as milk in a vessel said apparatuscomprising said vessel, a resistor having a conducting surface on aninsulating former rod disposed vertically in use in said vessel, saidresistor having an average perimeter in excess of approximately 63 mmand a resistance between three and thirty-two ohms per mm of length ofthe resistor, wherein said former rod is wound with wire having at leastthree turns per mm and a resistance of between one and four ohms perturn and being arranged so that increase in the level of liquid in thevessel causes some of the liquid to interconnect adjacent turns of saidwire which are immersed in the liquid and so that increase in the levelof liquid in the vessel causes interaction of the liquid in the vesselwith the resistor to reduce the impedance thereof, means for measuringsaid reduction in impedance, and display means responsive to changes inimpedance and on which change in level of liquid in the vessel isindicated. .Iaddend. .Iadd.14. Apparatus as claimed in claim 13 whereinsaid resistor has an average perimeter of less than 140 mm. .Iaddend..Iadd.15. Apparatus as claimed in claim 10, wherein said former rod ismade of a softenable material such as polysulfone or polycarbonate andsaid resistance wire is embedded in said softenable material of theformer with only a portion thereof exposed. .Iaddend.